These teachings relate generally to the generation of laser action in scattering media and, more specifically, to the use of random laser action for remote sensing and other applications.
Various physical phenomena associated with the multiple scattering of electrons have been known for some time. Over the past several years optical analogs of these phenomena have been observed, as reported by Dalichaouch, R., Armstrong, J. P., Schultz, S., Platzman, P. M. and McCall, S. L. Microwave Localization by 2-Dimensional Random Scattering. Nature 354, 53 (1991); Sparenberg, A., Rikken, G. and van Tiggelen, B. A. Observation of photonic magnetoresistance. Physical Review Letters 79, 757 (1997); and Scheffold, F. and Maret, G. Universal conductance fluctuations of light. Physical Review Letters 81, 5800 (1998).
In addition to these effects, it was predicted in 1968 that multiple scattering of light in the presence of amplification could lead to an instability and laser-like emission, Letokhov, V. S. Sov. Phys. JETP 26, 835 (1968). Such an experimental situation was realized in a number of physical forms included use of powdered laser crystals: Gouedard, C., Husson, D., Sauteret, C., Auzel, F. and Migus, A. Generation of Spatially Incoherent Short Pulses in Laser-Pumped Neodymium Stoichiometric Crystals and Powders. Journal of the Optical Society of America B-Optical Physics 10, 2358 (1993), Wiersma, D. S. and Lagendijk, A. Light diffusion with gain and random lasers. Physical Review E 54, 4256 (1996), and high gain laser dyes in combination with various scattering media: Lawandy, N. M., Balachandran, R. M., Gomes, A. S. L. and Sauvain, E. Laser Action in Strongly Scattering Media (Vol 368, Pg 436, 1994). Nature 369, 340 (1994). Most recently, experiments using zinc-oxide powders have provided evidence for random laser action in the presence of Anderson localization: Cao, H. et al. Random laser action in semiconductor powder. Physical Review Letters 82, 2278 (1999). Considerable experimental work on the physical mechanisms and possible uses of the random laser action followed the discovery of the dye based system. Examples of research into this area are found in the following articles: Balachandran, R. M., Perkins, A. E. and Lawandy, N. M. Injection Locking of Photonic Paint. Optics Letters 21, 650 (1996); de Oliveira, P. C., Perkins, A. E. and Lawandy, N. M. Coherent Backscattering from High Gain Scattering Media. Optics Letters 21, 1685 (1996); Balachandran, R. M. and Lawandy, N. M. Understanding Bichromatic Emission from Scattering Gain Media. Optics Letters 21, 1603 (1996). Notably, the inventor was a contributor to each of these articles. Potential applications include identification, remote sensing, displays, and photodynamic therapy, to name a few (see, for example, Balachandran, R. M., Pacheco, D. P. and Lawandy, N. M. Photonics Textile Fibers. Applied Optics 35, 1991 (1996); and Lawandy, N. M. xe2x80x98Paint-On Lasersxe2x80x99 Light the Way for New Technologies. Photonics Spectra 28, 119 (1994).
Reference can also be had to the following U.S. Patents, wherein the present inventor is either the sole inventor or a co-inventor: U.S. Pat. No. 6,100,973, Methods and apparatus for performing microanalytical techniques using photolithographically fabricated substrates having narrow band optical emission capability; U.S. Pat. No. 6,088,380, Method and apparatus for intracavity pixelated lasing projection; U.S. Pat. No. 6,030,411 Photoemitting catheters and other structures suitable for use in photo-dynamic therapy and other applications; U.S. Pat. No. 5,903,340, Optically-based methods and apparatus for performing document authentication; U.S. Pat. No. 5,448,582, Optical sources having a strongly scattering gain medium providing laser-like action; and U.S. Pat. No. 5,434,878, Optical gain medium having doped nanocrystals of semiconductors and also optical scatterers.
Motivated by the number of such applications, interest has grown in methods to externally control the random laser line-width, intensity, and emission wavelength. The wavelength can be controlled by mechanisms that affect the chromophore emission wavelength, while the line-width and intensity are most directly affected by the sample volume and the scattering length of the active medium. This was previously presented by the inventor and others: Balachandran, R. M., Lawandy, N. M. and Moon, J. A. Theory of Laser Action in Scattering Gain Media. Optics Letters 22, 319 (1997). Tuning of the output wavelength of a random laser has been demonstrated using a dye dissolved in a polymethylmethacrylate matrix. The 5 nm wide emission could be tuned by over 30 nm and was observed to be linear from 77K to 380K with a slope of approximately 0.09 nm/K, as described in International Patent Application No.: WO 00038283.
It has been proposed to use liquid crystals to control the properties of photonic bandgap crystals, as discussed by Busch, K. and John, S. Liquid-crystal photonic-band-gap materials: The tunable electromagnetic vacuum. Physical Review Letters 83, 967 (1999). This approach relies on the different optical properties of the various partially ordered liquid crystal phases that exist at various temperatures. The most dramatic effect on the scattering length occurs with the transition from the birefringent nematic phase to the isotropic phase. Unfortunately, the use of this approach for controlling a random laser is limited by the solubility of laser dyes in liquid crystal materials, requires a solid host structure and has a very small range of scattering length variation. This latter parameter is the critical factor in determining the threshold, linewidth, and output of the random laser.
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
A random laser system includes Critical Solution Temperature (CST) material, either a Lower CST (LCST) or an Upper CST (UCST) material, in combination with an optical gain medium, such as a laser dye. A laser-like emission is observed in response to optical pumping only when the CST material is in a scattering state. The random laser is suitable for being microencapsulated, and can be used for, as examples, remote temperature sending applications as well as for visual display applications.
A system is disclosed for creating a random laser media, where the scattering wavelength predictably relates to the temperature of the media. This invention may be used in a variety of applications, such as for the remote sensing of temperature. A preferred embodiment of this invention employs a LCST material as a scattering phase, in combination with an optical gain medium that includes a dye, such as a laser dye.
In a preferred, but non-limiting, embodiment of this invention a random laser action media is formed from a combination of hydroxypropyl cellulose (HPC) as the LCST material and the laser dye known as Kiton Red 620, where the laser dye is dissolved into the HPC. The resulting random laser action media exhibits a temperature dependent shift in scattering wavelength that is both predictable and reproducible.
In accordance with these teachings a random laser is provided within a container that contains an optical gain medium in combination with a LCST material. The random laser, when pumped by an external source, has an emission that exhibits laser characteristics only when a temperature within the container is above the LCST of the LCST material.
The container may be a capsule having a size of microns or tens of microns, and a plurality of such capsules can be disposed within a coating upon an object. In another embodiment an optical filter can be used, and the emission passes through the optical filter. The gain medium may be a laser dye, and the LCST material may comprise an aqueous hydroxypropyl cellulose (HPC) system.
A method is also disclosed for remotely sensing the temperature of an object. The method includes providing the object with a random laser action medium, where the medium comprises a laser dye and LCST material and exhibits a dependence of laser emission upon temperature; interrogating the object by irradiating the random laser action medium with a pump laser beam for inducing an emission from the random laser action medium; detecting the emission; and correlating at least one characteristic of the emission with the temperature of the object. The emission characteristic may include linewidth, emission strength and/or line center emission strength.
A display in accordance with an embodiment of this invention has a plurality of pixels, where each pixel includes an optical gain medium in combination with a CST material that, when pumped by an optical source, has an emission that exhibits a change in an emission characteristic as a function of temperature. The display further includes a thermal energy source for varying the temperature of each pixel individually to be above or below the CST of the CST material, depending on whether the pixel is intended to be on or off.
Also disclosed is a method for operating a random laser action medium. This method includes providing the medium so as to include an optical emitter material, such as a laser dye, and a CST material; illuminating the medium with light for causing the optical emitter to emit photons; and reversibly controlling the photon scattering length and the photon diffusion time in the medium in accordance with the temperature of the medium for controlling an optical emission characteristic of the medium. The optical emission characteristic can be one or more of linewidth, emission strength and line center emission strength.
Another embodiment of this invention employs the Upper Critical Solution Temperature (UCST) material as a scattering phase, in combination with an optical gain medium that includes a dye, such as a laser dye.